ABSTRACT This proposal requests an administrative supplement to enhance an active funded NICHD grant with an additional target population. The parent grant, R01-HD085833, is a randomized trial evaluating the impact of a social network intervention to increase regularity of HIV testing among young men who have sex with men (MSM) in Sofia, Bulgaria and to link HIV-infected youth to medical care. The parent study enters its fourth year on 7-1-18 and is proceeding well and according to its planned timeline. The parent trial has enrolled primarily young ethnic majority Bulgarian MSM. The supplement will add and expand the representation of young Roma (Gypsy) MSM to the intervention outcome trial sample. Roma are the largest and most marginalized ethnic minority population in Europe, and Roma suffer profound health disparities compared to the majority population due to impoverishment, unemployment, discrimination, and geographic segregation. A high proportion of young Roma men report high-risk sexual contacts with both males and females, but very few have had an HIV test or are in HIV medical care. Young Roma MSM are not a part of the visible MSM community and have limited mobility outside of Roma slum communities, necessitating the use of specialized field staff expertise dedicated to recruiting a Roma sample within the study. Activities with the Roma sample will be the same as those used in the parent grant and the study's specific aims are unchanged from the original proposal. During the past year, 67 adolescent and young Roma MSM were added to the parent grant's intervention outcome trial. However, this sample is not of sufficient size to permit analysis of intervention effects among ethnic minority MSM. The requested supplement will enhance representation of this additional population of youth by supporting the recruitment and randomization of 14 additional young Roma MSM social networks (n=approximately 98 participants) to the parent grant's experimental or comparison conditions. The intervention identifies, trains, and engages network influence leaders to encourage uptake of regular HIV testing by young MSM peers. Effects of the intervention are observed on testing behavior at 6- and 12-month followup, with corroboration by monthly health department records of HIV testing performed at Sofia VCT sites. Adding a young Roma MSM sample to the study will enable us to determine intervention effects on an underserved and understudied community population, and will contribute to the field's knowledge of approaches to increase regular HIV testing and care linkage among ethnic minority MSM.